It has been found that blood and blood components such as plasma and serum of both normal adults and of patients who have been administered antibiotics contain nonspecific bacteriostatic substance, i.e., bacterial growth inhibitor. This observation was made while conducting studies relating to an automated serum bioassay for drugs such as gentamicin, tobramycin, or amikacin in blood serum or plasma.
Typically, serum samples are assayed using a bacterial culture in growth medium, and the ability of the patient serum to inhibit bacterial growth is a measure of the level of antibiotic content in the serum specimen. It was unexpectedly found that about 50% of the human sera tested contained nonspecific bacterial growth inhibitor which caused the serum to give erratic bioassay results. In other words, bacterial growth could be inhibited irrespective of the antibiotic levels present in the serum or plasma being tested. It has been discovered that salicylates and closely related compounds will neutralize or inactivate these bacterial growth inhibitors present in blood or blood components.